1. Field of the Invention
The present invention relates to a polymer electrolyte membrane, and a membrane electrode assembly and a polymer electrolyte fuel cell using the polymer electrolyte membrane.
2. Description of Related Art
Fluorocarbon polymer electrolyte membranes having a high proton conductivity such as Nafion (registered trademark, supplied by Dupont), Aciplex (registered trademark, supplied by Asahi Kasei Chemicals Corporation), or Flemion (registered trademark, supplied by Asahi Glass Co., Ltd.) are known as polymer electrolyte membranes of fuel cells. However such fluorocarbon electrolyte membranes are very expensive and form hydrofluoric acid when they are burnt upon discarding.
In addition, the fluorocarbon electrolyte membranes are not usable at high temperatures of 100° C. or higher because of reduction in ionic conductivity. When used as electrolyte membranes in direct methanol fuel cells (hereinafter also referred to as DMFCs), they suffer from problems such as reduction in voltage and reduction in electric efficiency due to methanol crossover.
To solve these problems, Japanese Unexamined Patent Application Publication No. 2003-31232 and Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2006-512428 disclose hydrocarbon polymer electrolyte membranes composed typically of inexpensive poly (ether sulfone) or poly (ether ketone) polymers as polymer electrolyte membranes for use in polymer electrolyte fuel cells and direct methanol fuel cells. These are proposed as alternatives for fluorocarbon electrolyte membranes.
In fuel cells, there occurs a phenomenon in which a peroxide is formed in an electrode catalyst layer as a result of an electrode reaction, the peroxide diffuses and becomes a free radical, and the resulting peroxide free radical erodes and deteriorates the electrolyte. The generation of the peroxide free radical is promoted particularly by metal ions (e.g., Fe2+ and Cu2+) dissolved out from a charging line of mist which is to be mixed with a feed fuel (gas or liquid) so as to keep the feed fuel or electrolyte wet. For these reasons, the polymer electrolyte membranes composed of poly(ether sulfone) block copolymers or poly(ether ketone) block copolymers which do not having so satisfactory oxidation resistance are oxidized, decomposed and deteriorated by the peroxide free radical, resulting in short lives.
Japanese Unexamined Patent Application Publication No. 2005-216701 and Japanese Unexamined Patent Application Publication No. 2005-353408 disclose techniques of forming a layer which contains a metal oxide serving as a peroxide-decomposing catalyst between an electrode catalyst layer and an electrolyte layer in order to reduce the deterioration of the electrolyte membrane.
Japanese Unexamined Patent Application Publication No. 2002-110174 discloses a polymer electrolyte composed of an aromatic hydrocarbon polymer compound including a sulfoalkyl group in its side chain.
Japanese Unexamined Patent Application Publication No. 2007-134306 discloses a membrane electrode assembly for a direct oxidation fuel cell which includes an anode and a cathode each provided with a catalyst layer containing a solid proton-conductive material and a catalyst, in which the proton-conductive material contained in the anode catalyst layer has an equivalent weight (EW) larger than the equivalent weight (EW) of the proton-conductive material contained in the cathode catalyst layer.
Japanese Unexamined Patent Application Publication No. 2002-164057 discloses a polymer electrolyte fuel cell which includes a polymer electrolyte membrane and two catalyst layers provided on both principal surfaces of the polymer electrolyte membrane, in which the catalyst layers each include a catalyst powder and at least two different ion-exchange resins having different EWs, and the EWs of the ion-exchange resins in the catalyst layers vary along the thickness direction and plane direction of the catalyst layer.
Japanese Unexamined Patent Application Publication No. 2005-317287 discloses a membrane electrode assembly which includes electrodes each having a catalyst layer; and an electrolyte containing a proton-conductive resin, in which the catalyst layer is immersed with the electrolyte, and the proton-conductive resin in the electrolyte has an EW gradually increasing from the upstream side toward the downstream side of gas passages in contact with the electrodes.
Japanese Examined Patent Application Publication No. H01-52866 discloses a fuel cell having satisfactory ionic conductivity, which is composed of a hydrophilic membrane having sulfone groups in an amount in terms of exchange capacity of 0.1 to 3 milliequivalents per gram, having an electric resistance of 0.05 to 2 Ω·cm2 in a diluted sulfuric acid, and having a methanol permeability coefficient of 5×10−3 cm/min or less. This patent literature also describes a method for measuring an exchange capacity.